1. Field of the Invention
The present invention relates to a process for preparing acrylic acid by heterogeneously catalyzed partial gas phase oxidation of propylene, by    a) in a first reaction stage, subjecting propane to a homogeneous and/or a heterogeneously catalyzed dehydrogenation and/or oxydehydrogenation in the presence of and/or with exclusion of oxygen to obtain a product gas mixture 1 comprising propane and propylene, and    b) if appropriate removing and/or converting to other compounds a portion of the constituents, other than propane and propylene, present in the product gas mixture 1 formed in the first reaction stage to obtain a product gas mixture 1′ from product gas mixture 1, and    c) subjecting product gas mixture 1 and/or product gas mixture 1′, as a constituent of a starting reaction gas mixture 2 which comprises molecular oxygen and propylene in a molar O2:C3H6 ratio of ≧1, to a heterogeneously catalyzed partial gas phase oxidation of propylene present in product gas mixture 1 and/or product gas mixture 1′ to acrolein in a second reaction stage charged with a fixed catalyst bed 2 whose catalysts have at least one multimetal oxide comprising the elements Mo, Fe and Bi as an active composition to obtain a product gas mixture 2, and    d) if appropriate lowering the temperature of the product gas mixture 2 leaving the second reaction stage by indirect and/or direct cooling and if appropriate adding molecular oxygen and/or inert gas to product gas mixture 2, and    e) then subjecting it, as a starting reaction gas mixture 3 which comprises acrolein, molecular oxygen and at least one inert gas and comprises molecular oxygen and acrolein in a molar O2:C3H4O ratio of ≧0.5, to a heterogeneously catalyzed partial gas phase oxidation of acrolein present in starting reaction gas mixture 3 to acrylic acid in a third reaction stage charged with a fixed catalyst bed 3 whose catalysts have at least one multimetal oxide comprising the elements Mo and V as an active composition to obtain a product gas mixture 3, and    f) removing acrylic acid in a separating zone A from the product gas mixture 3 and recycling at least the unconverted propane and propylene present in product gas mixture 3 to an extent of in each case at least 80 mol % based on the particular amount present in product gas mixture 3 into at least the first of the three reaction stages.
2. Driscription of the Background
Acrylic acid is an important monomer which finds use as such or in the form of its alkyl esters for obtaining polymers suitable, for example, as adhesives.
It is known that acrylic acid can be prepared by two-stage heterogeneously catalyzed partial gas phase oxidation of propylene (cf., for example, EP-A 990 636, U.S. Pat. No. 5,198,578, EP-A 10 15 410, EP-A 14 84 303, EP-A 14 84 308, EP-A 14 84 309 and U.S. 2004/0242826).
It is characteristic of the aforementioned processes that the acrylic acid is not obtained as such, but rather as a constituent of a product gas mixture from which it subsequently has to be removed.
It is common to substantially all separation processes known in this regard that, if appropriate after direct and/or indirect cooling of the aforementioned product gas mixture, acrylic acid present in the product gas mixture is converted to the condensed phase in a basic removal step. This may be effected, for example, by absorption into a suitable solvent (for example water, high-boiling organic solvents, aqueous solutions) and/or by partial or substantially full condensation (for example by fractional condensation) (on this subject, cf., for example, the abovementioned documents, and the documents EP-A 13 88 533, EP-A 13 88 532, DE-A 102 35 847, EP-A 79 28 67, WO 98/01415, EP-A 10 15 411, EP-A 10 15 410, WO 99/50219, WO 00/53560, WO 02/09839, DE-A 102 35 847, WO 03/041833, DE-A 102 23 058, DE-A 102 43 625, DE-A 103 36 386, EP-A 85 41 29, U.S. Pat. No. 4,317,926, DE-A 198 37 520, DE-A 196 06 877, DE-A 19 50 1325, DE-A 102 47 240, DE-A 197 40 253, EP-A 69 57 36, EP-A 98 22 87, EP-A 10 41 062, EP-A 11 71 46, DE-A 43 08 087, DE-A 43 35 172, DE-A 44 36 243, DE-A 19 924 532, DE-A 103 32 758 and DE-A 19 924 533). An acrylic acid removal may also be undertaken as described in EP-A 98 22 87, EP-A 98 22 89, DE-A 103 36 386, DE-A 101 15 277, DE-A 196 06 877, DE-A 197 40 252, DE-A 196 27 847, EP-A 92 04 08, EP-A 10 68 174, EP-A 10 66 239, EP-A 10 66 240, WO 00/53560, WO 00/53561, DE-A 100 53 086 and EP-A 98 22 88. Favorable modes of removal are also the processes described in the documents WO 2004/063138, WO 2004/035514, DE-A 102 43 625 and DE-A 102 35 847.
The condensed phase which comprises acrylic acid and is obtained in the basic removal described may then be further purified, for example, by extraction, rectification, desorption and/or crystallization to the desired degree of purity of the acrylic acid. For example, the further purification may be effected as described in the documents WO 01/77056, WO 03/041832, WO 02/055469, WO 03/078378 and WO 03/041833.
A common feature of the basic removals described is that a residual gas stream (cf. also EP-A 11 80 508) normally remains (typically at the top of the separating column which comprises separating internals and is used for the basic removal), which comprises substantially those constituents of the product gas mixture whose boiling point at standard pressure (1 bar) is ≦−30° C. (i.e. the constituents which are difficult to condense or else highly volatile). The residual gas constituents are primarily reactants unconsumed in the partial oxidation, i.e. molecular oxygen (is generally used in excess relative to the stoichiometry of the partial oxidation in order to modify the catalyst activity advantageously) and in some cases propylene, and in particular inert diluent gases used in the partial oxidation, for example nitrogen, noble gases, carbon oxides and saturated hydrocarbons. Depending on the separation process employed, steam may be present in the residual gas only in traces or in amounts of up to 20% by volume, or up to 25% by volume or more. In small amounts, the residual gas may also comprise acrylic acid and/or acrolein which are normally, however, predominantly basically removed as described.
A disadvantage of the remaining residual gas is that it can be recycled into the partial oxidation only in a limited amount. More substantial recycling is not possible because the cycle gas constituents would otherwise accumulate in such a cycle gas mode. This would have the consequence of reaction gas streams in the partial oxidation which are no longer manageable and finally the extinguishing of the partial oxidation. In general, at most half of the residual gas is therefore recycled as oxidation cycle gas into the partial oxidation and the remaining amount is sent to combustion as outlet gas (cf., for example, EP-A 925 272).
Among other reasons, the aforementioned is disadvantageous because propylene which has not been converted in the partial oxidation and remains in the residual gas is necessarily lost (a propylene removal from the residual gas and subsequent separate recycling of the removed propylene into the partial oxidation is of low economic viability owing to the small fractions of propylene).
The disadvantageousness is so great in particular because the starting propylene used for the acrylic acid preparation is normally propylene having a comparatively high purity (for example polymer-grade or chemical-grade propylene; cf. DE-A 101 31 297), which has normally passed through a separation from the other hydrocarbons, for example propane, which accompany it as a result of the preparation.
For this reason among others, very high propylene conversions are therefore pursued in the first reaction stage (cf. WO 03/029177). The same also applies to the acrolein conversion in the second reaction stage. This is the case additionally because acrolein which has not been converted in the second reaction stage becomes noticeably disadvantageous, for example, in the removal of acrylic acid from the product gas mixture of the partial oxidation (undesirably promotes the polymerization tendency of acrylic acid (cf., for example, DE-A 10 2004 021 763 and DE-A 10 2004 021 706)). In order to obtain the aforementioned high conversions, the prior art even recommends the use of postreactors (cf., for example, DE-A 10 2004 021 763 and DE-A 10 2004 021 706).
In particular on the industrial scale of the two-stage partial oxidation of propylene to acrylic acid, propylene conversions of >99.5 mol % are in many cases pursued in the first reaction stage in order to be able to fully dispense with an oxidation cycle gas recycling into the partial oxidation. Such an operating mode additionally includes the advantage that the energy required for the oxidation cycle gas recycling (the oxidation cycle gas has to be recompressed to partial oxidation pressure by means of a turbocompressor before the recycling) and the not inconsiderable investment in the compressor (for example one of the model 12 MH4B from Mannesmann DEMAG, Germany) is not required.
However, high conversions typically require catalysts which are highly tuned with regard to their activity and/or high temperatures in the partial oxidation. Both have a disadvantageous effect on the catalyst lifetime (cf. DE-A 103 51 269 and DE-A 10 2004 025 445). This is the case in particular because the reaction gas mixture passes through a maximum value, known as the hotspot value, as it flows through the fixed catalyst bed in each of the two partial oxidation stages.
For reasons of convenience, the temperature of the fixed catalyst bed and the effective temperature of the fixed catalyst bed are therefore distinguished from one another. The temperature of the fixed catalyst bed refers to the temperature of the fixed catalyst bed when the partial oxidation process is being performed, but in the theoretical absence of a chemical reaction (i.e. without the influence of the heat of reaction). In contrast, the effective temperature of the fixed catalyst bed refers to the actual temperature of the fixed catalyst bed including the heat of reaction of the partial oxidation. When the temperature of the fixed catalyst bed is not constant along the fixed catalyst bed (for example in the case of a plurality of temperature zones), the term temperature of the fixed catalyst bed means the (numerical) average of the temperature along the fixed catalyst bed. The temperature of the fixed catalyst bed over its length may of course also be configured in such a way that the temperature is constant over a certain length, then changes abruptly and maintains this new value over a further length, etc. In that case, reference is made to a fixed catalyst bed (fixed bed catalyst charge) having more than one temperature zone (or else reaction zone), or disposed in more than one temperature zone (or else reaction zone). Catalyst-charged reactors which implement such temperature zones (or else reaction zones) are correspondingly referred to as one-zone or multizone reactors (cf., for example, WO 04/085369). In common with the temperature of the reaction gas mixtures, the effective temperature of the fixed catalyst bed likewise passes through the hotspot value in flow direction of the reaction gas mixture.
It is possible to make use of the possibility of lowering the hotspot temperature by operating the partial oxidation process at reduced reactant conversion in the conventional, above-described propylene partial oxidation to acrylic acid at best with considerable disadvantages for the above-described reasons.